1. Field of the Invention
The present invention relates to a conductivity-modulated MOSFET of the type which has recently attracted attention as an insulated gate bipolar transistor (hereafter abbreviated as the IGBT).
2. Discussion of the Related Art
A typical n-channel IGBT is shown in FIG. 2. In the IGBT shown in FIG. 2, the drain region of a vertical MOSFET, for handling electric power, is the opposite conductivity type as the source region. A p-type region 4 is selectively formed within a surface layer of an n-type base layer 3 which is on a p.sup.+ -type drain layer 1 and an n.sup.+ -buffer layer 2. An n-type source region 5 is selectively formed within a surface layer of the p-type region 4. A channel forming region 41 is sandwiched between the n-type layer 3 and the n-type source region 5 in the p-region 4. A gate 6, of polycrystalline silicon, is provided over the surface of the channel forming region 41. An insulating film is between the gate 6 and the channel forming region 41. A source electrode 7 contacts the p-type region 4 and the n-type source region 5, while a drain electrode 8 contacts the p.sup.+ -type drain layer 1. The p-type region 4 of the IGBT is fabricated by the introduction of impurities into the n.sup.- -type base layer 3 using the gate 6 as a mask, and the channel forming region 41 is fabricated using a self-alignment technique.
The operation of the IGBT will now be described. As a voltage is applied to the gate 6, the portion of the p-type region located immediately below the gate becomes an n-inversion layer, and a channel is formed in the channel forming region 41. Electrons flow from the source region 5 through the channel forming region 41, and are injected into the base layer 3. Correspondingly, positive holes are injected into the base layer 3 from the drain layer 1 via the buffer layer 2, and conductivity modulation occurs. Because the on voltage of an IGBT does not rise in light of its basic principle, unlike a power MOSFET, IGBT's have been used extensively in high-voltage applications.
A conventional conductivity-modulated MOSFET device is shown in FIG. 3. As shown in FIG. 3, a p-type layer 40 is in an outermost periphery 9 of the device chip. This layer is usually formed simultaneously with the p-type layer 4 by the introduction of impurities from outside a thick oxide film 11.
The source region 4 contacts the source electrode 7 connected to a source terminal S which is normally at the ground potential. When the device is off, a depletion layer spreads from the source region 4 toward the drain layer 1 having the drain potential. At the same time, the depletion layer also spreads toward an outer periphery 9 of the chip which is at the drain potential. An electrode 10 is formed in contact with the p-type region 40 to stop the spread of the depletion layer.
In a high-voltage withstanding device, a material having a high specific resistance is used for the n.sup.- -type layer 3 because of the avalanche withstand voltage. For this reason, the depletion layer increasingly spreads in the horizontal direction and the electrode 10 is unable to stop the spread of the depletion layer. Hence, the avalanche withstand capacity decreases undesirably.
Because of this problem, the stopper effect of the aforementioned electrode 10 is not reliable, and a parasitic transistor is formed in the horizontal direction. When the parasitic p-n-p transistor consisting of the p-type layer 4, the n.sup.- -type layer 3, and the p-type layer 40 operates, the avalanche withstand capacity is undesirably decreased. The depletion layer spreads widely because of the high specific resistance of the n.sup.- -type layer 3, and the remaining width of the base layer of the parasitic p-n-p transistor becomes increasingly short, creating a narrow-base transistor having a high efficiency. As a result, the parasitic p-n-p transistor is operated, undesirably decreasing the avalanche withstand capacity.
When the conventional IGBT is operated, a voltage is applied across the source and the drain. As shown in FIG. 4, there is a voltage range between 0 and V.sub.0 volts called a blocking-layer voltage where current does not flow. The voltage V.sub.0 is approximately 1V. The fact that a voltage range exists where current does not flow despite the application of voltage across the source and drain is a problem when the IGBT is used as a switching device in a voltage resonance circuit. In that application, even a small current flow after the application of a voltage across the source and drain would be acceptable. However, because current does not practically flow and current starts to flow when the voltage has risen slightly, the value of di/dt changes substantially in the voltage resonance circuit. Accordingly, in a coil L connected to the circuit, a voltage .DELTA.V=Ldi/dt occurs, which becomes undesirable noise.